Multiple stressors and amphibian population declines

Abstract:

In the past decade, declines in amphibian populations have captured scientific and
popular interest. The causes of the declines are likely to be complex and involve
interactions between several environmental stressors. Using multifactorial
experiments, I investigated the combined effects of several anthropogenic stressors
on developing amphibians in Oregon, USA. In laboratory experiments, I found that
low levels of pH and exposure to high levels of nitrate killed larval Rana cascadae.
Moreover, exposure to UV-B radiation and exposure to high levels of nitrate
reduced larval activity level. Results suggest that in some cases, the effect of
increasing nitrate level on larval activity depended on the pH level. In outdoor
experiments, I investigated the combined effect of UV-B and nitrate fertilizer on
two species of amphibians at both low and high elevation sites in Oregon. In Hyla regilla, I found that UV-B and nitrate together had a negative effect on larval mass
in the lower elevation site but adversely affected survival in the high elevation site.
Nitrate increased larval mass in Ambystoma macrodactylum. However, in the
higher elevation experiment, this effect occurred only when UV-B was blocked.
Finally, using both laboratory and outdoor experiments, I investigated the
combined effects of UV-B with the two commonly used pesticides, carbaryl and
chlorpyrifos. I studied these effects in the larvae of three species of amphibians
native to the highly agricultural Willamette Valley in Oregon: Rana aurora, A.
macrodaclylum, and H. regilla. Laboratory results for all three species revealed that
a formulated pesticide product of chlorpyrifos killed larvae although the active
ingredient alone did not, suggesting that some components of the pesticide
formulations could be toxic to larval amphibians. In the laboratory study, there
were no adverse effects caused by UV-B. However, outdoor studies indicated that
ambient levels of UV-B enhance pesticide toxicity in amphibians. Both pesticides
were toxic to A. macrodaclylum in the presence of UV-B. Chiorpyrifos caused
mortality in R. aurora in the presence of UV-B. Overall, results demonstrate the
importance of considering multiple environmental stressors together in assessing
amphibian population declines.